To the Editor.—Angiotensin-converting enzyme (ACE) inhibitors, widely prescribed for the treatment of hypertension, have become a mainstay in the treatment of congestive heart failure. These agents may increase serum potassium, with an incidence of 1% to 2.2% in patients treated for hypertension, and 3.5% to 4.8% in patients treated for congestive heart failure.1 Advanced renal disease, diabetic nephropathy, potassium supplementation, and potassium-sparing diuretics all increase the risk of hyperkalemia in patients taking ACE inhibitors.2,3 Many patients with cardiac conditions or elevated blood pressure are counseled to use salt substitutes and to increase their oral potassium intake by ingestion of potassium supplements or foods rich in potassium. Patients may not realize that potassium supplementation can be problematic when taking ACE inhibitors, and physicians may not be aware that their patients are receiving additional potassium, either through diet or supplementation.We surveyed patients taking ACE inhibitors found to have